The relates to a method for authentication by an external medium of a portable object, such as a memory card coupled to this medium.
More particularly, the invention applies to a case where the function of the external medium is to furnish a service or authorize access to a system via a card, but where, knowing that only some cards are authorized to cooperate with this external medium, the external medium must first determine that the card coupled to it has indeed been authorized from the outset rights to access this service.
Generally, the majority of applications using portable objects such as cards take into account at least one secret key that is specific to each application. On the one hand, this key is prerecorded in all the cards that are allowed to have access to this application, and on the other hand in all the items of equipment with which these cards can be connected in order for a service to be furnished or access authorized within the context of this application. These secret keys are known only to the authorizing entities that issue the cards and oversees the equipment that cooperates with these cards. In each of these applications, the equipment must first verify that the card indeed has the same secret key as its own, without moreover divulging in any way what the value of this key is.
This verification is typically effected by an exchange of information between the card and the equipment in the form of a dialogue, such as is described in French Patent 2 469 760 and corresponding U.S. Pat. No. 4,549,075 to Saada et al, for example.
According to this patent, the dialogue comprises externally inputting a random number into the card; causing the processing circuits of the card to calculate a result which is at least a function of this random number and of a secret key prerecorded in the card; extracting this result from the card; and comparing it with a result calculated by the equipment, which is at least a function of this same random number and of a secret key prerecorded in the equipment.
This verification or authentication of the card is confirmed, if the two results agree. It is understood that this agreement cannot be obtained unless the secret keys are identical.
To prevent any defrauder from making counterfeit cards, the secret nature of the keys used must absolutely be maintained, especially in the equipment that is accessible by the public, such as terminals. Generally, these keys are recorded in a memory zone which is then locked to prevent access to it from the outside, this zone being accessible only internally, by the processing circuits connected to this memory. However, it must be acknowledged that fraud, even though limited and technically difficult, is always possible, and that a defrauder will seek to discover the secret keys by sophisticated techniques, such as laser reading of the memory containing the keys.